Bile acids and their salts have a complex physiological function in lipid absorption and the formation and dissolution of gallstones. This function can be correlated in a qualitative manner with their detergent-like properties. Like other hydrophobic solutes, bile salts are capable of hydrophobic bonding. They self-associate, and form mixed aggregates with a variety of lipids. Recent research by the author and others has revealed the importance of the molecular structure of hydrophobic solutes in producing different patterns of association, and micellar solubilization. The main objective of this research is to investigate the role of the unique structure of bile salts, and to what extent these physiological surfactants differ from ordinary detergents. The customary assumption that they behave like micelle-forming surfactants is crude and often misleading. Many of the established approaches for studying detergents are inapplicable or insensitive for bile-salt systems. Two new partition methods developed by the author for fatty acids and dyes will be adapted for studying the self-association of the bile salts. The solubilization of physiological lipids such as fatty acids, glycerides, lecithin, or cholesterol will be studied in a quantitative manner in terms of mutual association. Some spectroscopic probes developed by the principal investigator and his coworkers will be used to investigate the disposition of suitable aromatic solubilizates in the aggregates, and the polarity of the micro-environment and states of ionization at the interface of bile salt-lipid aggregates. These in-vitro studies should provide the basis for posing and answering suitably focused questions related to the physiology and patho-physiology of biliary systems at a later time.